There exist today several innovative manufacturing technologies to meet the demand for the production of components with features in the range of a sub-micron to several hundred micrometers. They are classified into two basic groups (Rajurkar et al, 2006): (i) lithography based micro fabrication processes, which are capable of micro and sub-micrometer size features, and (ii) micro manufacturing processes, which are capable of micro and miniaturized part fabrications. Unfortunately, we are rapidly approaching the limit of traditional processing methods for functionalizing and processing inexpensive miniaturized devices. Clearly, a major challenge remains in the micro-manufacturing community to develop flexible, robust and large-scale fabrication methods that are economical and also environmentally friendly.
Traditionally, the lithography based processes employ either material addition (e.g., Physical Vapor Deposition “PVD,” Chemical Vapor Deposition “CVD,” and electro-deposition) or material subtraction (e.g., UV and e-beam lithography) to produce micron and submicron scale surface patterning. However, such processes are naturally limited by macro-scale phenomenon such as diffusion or thermal gradients. Any patterning scheme utilizing deposition methods must create and maintain a tight gradient (in the nanometer range) in driving force to control deposition and transport rates. Although subtraction processes such as electron or ion beam writing possess very high resolution capabilities for local patterning, they are sequential and cumbersome (due to macro-scale positioning requirements) with limitations in the materials they can modify and strict requirements of surface planarity. Thus, direct extension of lithographic based fabrication facility, with its attendant high cost of ownership (COO) and the required capital outlay of upwards of $3 billion are somewhat impractical for miniaturized components for targeting inexpensive and rapid throughput.
For example, the MicroStepper described by Miller et al. (2000) can achieve sub-100-nm patterning, but the equipment is expensive and requires planarized surfaces with roughness of the order of 0.1 times the wavelength of the ultraviolet light.
Non-lithographic based processes can also be classified as additive and subtractive processes (see Rajurkar et al., 2006 and the references therein for an exhaustive list of processes). Out of this list, we focus our attention on mechanical micromachining vs. electro-physical and chemical processes (ECP). In mechanical micromachining, a direct contact with the work piece is established, with good geometric correlation between the tool path and the work piece. While they possess high material removal rate, these methods, however, are not suitable for very hard or very fragile, e.g., low dielectric porous materials. In addition, they induce significant level of residual stresses, and possess additional limitations on dimensional tolerances and minimum gage requirements (Liu et al, 2004). On the other hand, ECP offer distinct advantages by not contacting the work-piece, especially in electro-discharge machining (EDM) and electrochemical machining (ECM). The ECP eliminates the drawback due to elastic spring back and the minimum gage requirement to sustain the cutting forces. In addition, they are quite economical for small batch productions (IWF, 2002). The ECP processes have been successfully employed in aerospace, automobile, and other industries for shaping, cutting, debarring and finishing. These processes provide solutions for manufacturing small and very precise components and micro-systems for the watch industry, micro-optics (telecommunications), medicine (processing biocompatible materials, medical implants) and chemical industry (micro-reactors).
ECM process can provide excellent performance for large and contoured surfaces. It also provides low material waste and very little tool wear. Complex shapes ranging from hard to machine titanium and wasp alloys aircraft engine casings (McGeough, 1974), to miniaturized LIGA processes are common utilization of ECM (Friedrich et al., 1997; Dunkel et al., 1998, Craston et al., 1988; Husser et al., 1989). While the EC process has found major applications in IC fabrications such as in Damascene Cu Plating (Andricacos, 1999) and in electrochemical mechanical planarization of wafers (Steigerwald et al., 1997; Huo et al., 2004), most ECM processes, however, are not environmentally benign. They also give rise to thermal and environmental concerns. The finished surface comes in contact with corrosive chemicals, which may accelerate corrosion and necessitate post-ECM cleaning of the finished surface (Wilson, 1971). Maintaining an ECM tool over a long period of time has also proved difficult.
The electrochemical process described by Mazur et al. (2005) is environmentally benign. However, it is only meant for polishing or planarization, and cannot imprint a specified pattern on a surface.
The traditional lithography or other contact printing processes also require extremely tight tolerances in surface roughness and planarization. This makes surface preparation for such processes quite expensive, often requiring chemical mechanical planarization “CMP.”
Thus, capability for printing on wavy surfaces is also required for flexible IC devices, where performing CMP is very difficult. Therefore, there is a need in the industry for a device that produces sub-100-nm patterns through a non-contact process. The conventional available devices that can produce such patterns are expensive, and also typically require polished or planarized surfaces.